1. Technical Field
The present invention relates to the processing of flat panels, integrated circuits or other wafer-like objects in a non-contact holder and, more particularly, to apparatus and methods for preventing unwanted motions of the object within the holder.
2. Description of Related Art
Wafer-like objects (hereinafter referred to as xe2x80x9cwafersxe2x80x9d) often require a means to hold the objects in position while performing processing or other operations thereon. Non-contact wafer holders may make use of a cushion of gas (typically air) to maintain a separation between the holder and the object being processed. Therefore, motion of the object in the plane of the holder is virtually frictionless, unless forces in addition to the air cushion are supplied. Holding the object in position in a non-contact holder while avoiding unwanted and possible dangerous motions in the plane of the holder, are among the primary goals of the present invention.
To be definite, we will describe the common instance of the processing of semiconductor wafers as may typically occur in the production of integrated circuits or in wafer post-processing for thinning, etc. However, the processing of any wafer-like object subject to unwanted motion while in a non-contact holder is also a potential area of application for the present invention. Flat panel displays and other rectangular, circular, star-shaped or irregularly shaped planar objects may require mounting in a holder for processing without the risk of damage to the object by contact with the holder. For economy of language we will refer to all such wafer-like objects herein as xe2x80x9cwafersxe2x80x9d recognizing that such objects may be large (perhaps having a size up to the order of one meter or larger) and need not be rectangular, circular or regular in shape. Since semiconductor wafers are expected to be an important area of application for the present invention, we will describe the primary features of the present invention in terms of semiconductor processing, not intending to limit the invention to this particular choice or particular example. Semiconductor wafers aptly illustrate the features of the present invention and permit obvious modification for use in processing other wafer-like objects.
Integrated circuits are typically fabricated on a wafer of silicon with numerous integrated circuits fabricated on a single wafer. This process requires the wafer to be held in position for the multiple processing steps required to complete circuit fabrication and for processing steps following fabrication of the integrated circuits. However, intimate mechanical contact between the wafer and its holder incurs the risk of damage to the wafer or the devices fabricated on the wafer""s surface. This risk of damage increases as wafers become thinner and easily distorted during processing. Thus, wafer holders that do not require mechanical contact between wafer and holder have been developed.
The floating of a wafer above a layer of compressed gas is described in the work of Pirker (U.S. Pat. No. 5,896,877). The wafer is held in position by gravity while the air cushion prevents contact with the wafer holder. Work of Siniaguine and Steinberg (PCT International Publication No. WO 97/45862) describes a non-contact holder for wafer-like objects in which a vortex of rotating air provides both the vacuum support for the wafer and the air cushion.
When the wafer holder is at rest during loading or unloading, the wafer is not stable in the holder. Non-contact wafer holders necessarily provide forces positioning the wafer in a direction perpendicular to the holder. Gravity is used in the work of Pirker while a vortex-created vacuum is used by Siniaguine et. al. However, both approaches to non-contact wafer positioning provide minimal hindrance to the wafer""s motion parallel to the surface of the holder. Random forces deriving from the use of an air cushion, or from small rotations of the holder during start-up and shut-down, may result in torques being applied to the wafer. Such torques may lead to undesirable motion of the wafer in its holder and the possibility of wafer damage.
Integrated circuits are typically fabricated in a multi-wafer holder or xe2x80x9ccarousel,xe2x80x9d rotating through a plasma for thinning. When the carousel containing many wafers begins to rotate, centrifugal forces press each wafer against limiting pins. A moving wafer contacting limiting pins may chip or perhaps fracture. Damage is more likely if the wafer is thin, typically about 50 microns. Procedures and devices for preventing such unwanted wafer motions during loading and unloading of the carousel are an example of the applicability of the present invention.
Non-contact wafer holders may use vacuum or gravity to keep the wafer in close proximity to the wafer holder while maintaining a separation by means of a gas or air cushion. However, undesired motion of the wafer in the plane of the wafer holder may build up to dangerous rates if not prevented. As wafer holders in a processing carousel rotate about a central axis of the carousel, centrifugal forces are commonly used to hold the wafer firmly against a limiting device, preventing unwanted separate motion of the wafer in its holder. However, after loading of the wafer into the holder but before centrifugal forces arise due to carousel rotation, the wafer may still build up dangerous levels of in-holder motion. Similar risks are encountered during wafer unloading. The present invention comprises a dynamic brake that applies restraining frictional force to a wafer in a wafer holder while the wafer holder is substantially at rest, but releases the restraining force as the processing carousel containing several wafer holders rotates about a central axis of the carousel. This dynamic brake preferably comprises a boot that passes through an opening in the wafer holder to rest on the surface of the wafer in an exclusion zone near the wafer""s edge. The exclusion zone is typically no more than about 3mm in extent. The frictional force between the boot and wafer is sufficient to prevent unwanted motions of the wafer in the holder. As the wafer holder rotates about the central axis of the processing carousel, centrifugal forces applied to the brake arising from such rotation cause the boot to pivot upward, releasing the frictional force on the wafer as the same centrifugal forces cause the wafer to press firmly against the limiting pins.